A wide range of industries utilize pipe and vessels comprised of fiber reinforced thermosetting resin material. Such materials are commonly known in the trade as fiberglass reinforced plastic ("FRP") materials, as glass reinforced plastic ("GRP") materials, or simply as reinforced thermosetting plastic ("RTP") materials, although the reinforcing fibers are often composed of materials other than glass, such as polyester, carbon, mixed boron, kelvar, etc. For convenience, the term reinforced thermosetting plastic, or the abbreviation "RTP," will be used herein, however, it is to be understood that the present invention is applicable to each of the above described materials, and to materials with similar molding and forming characteristics, through a range of variation in resin or resin-like formulations and reinforcing materials. Reinforced thermosetting plastic RTP materials are selected for service because of the need for materials which are corrosion resistant and which have adequate strength. Typical uses for corrosion resistant RTP equipment include pipe headers and drying towers (as used in chlorine manufacture), generators (as used in chlorine dioxide units), process reactors and piping (such as for fertilizer plants and chemical plants), scrubbers, coolers and vaporizers (as used in sulfuric acid plants), evaporators, waste tanks, recovery tanks (as used in mining or chemical manufacturing), cells, tanks, and recovery units (as in tin, lead, copper, or other metal extraction plating or purification units), washer hoods and bleach towers (as used in wood pulping), or as in chimney liners (as used in power plants and incinerators), petroleum processing, water treatment plants, and a variety of other uses.
Reference will on occasion be made below to the joint or to the juncture between a main pipe header or corresponding structure and a branch pipe part or similar structure. This is done simply for the sake of convenience and clarity and is not intended to limit the scope of my invention which is, in general, applicable to fabrication of any structure having angularly intersecting fluid containing portions. Also, directions of up, down, top, bottom, etc. are used for convenience and may correspond to illustrations provided, however, it is to be understood that the present invention provides a method and apparatus equally applicable to fabrication of parts at any desired branch pipe part orientation. In illustrations herein, nozzles may be shown in a direction that is perpendicular to the longitudinal axis of a main pipe part, however, the method of the present invention is adaptable to nozzle fabrication at any desired angle by creating a suitable mold shape and by including an angle in the mold support means designed for positioning a mold at the proper angle relative to the main pipe part. In addition, it is to be noted that the terms interior and exterior are used only for convenience in providing an explanation of the present invention. Such use is for illustrative purposes and in no way limits the scope of the present invention. There are applications where nozzles are desired on the "interior" rather than the "exterior," and where the method of fabrication taught by the present invention is particularly useful. Examples include but are not limited to applications such as "through hull" fittings for boats, ships, marine bulkhead applications, and aircraft of composite or resin-like materials of construction, or the like.
In structures of the character under consideration, a problem arises at the junctures between the main pipe part and an attached nozzle. Unlike metal pipe parts, attachments of RTP parts cannot simply be welded together and still provide adequate strength and corrosive resistance at such juncture or corner. In many different types of service, there is a need for improved corrosion resistance in equipment of RTP construction. In particular, there exists a need for improved corrosion resistance at joints and at nozzle connection points in RTP equipment.
In vessels constructed according to presently practiced methods, it is common for the initial failure point in RTP vessels and pipe systems to be the point of attachment, or joint, between the vessel and a nozzle which has been attached during fabrication.
In the fabrication of RTP equipment there are various methods known for attachment of nozzles to pipes or vessels. A traditional method of fabrication is known as the "tee insert" connection. The header is prepared by cutting a hole therein, the nozzle placed against the header or vessel and the parts joined by using a secondary overlay wrap of resin impregnated glass cloth.
In a similar, but slightly improved method, edges of parts to be joined are beveled so that bevels on the nozzle and on the vessel match. These matched surfaces are coated with resin and then fitted together. There are a number of disadvantages to this method. It is fairly difficult to achieve an exact fit between the parts unless the beveled angle can be precisely manufactured. Since manufacturing of such joints is usually done by manual grinding of the end of the nozzle and of the edge of the hole in the pipe or vessel, it is a normal result of such method of manufacture that voids are created which require filling. The filler is subject to corrosion and erosion and the result is low corrosion resistance in the joint between the two pipe parts. Also, the joint is sharp and angular. Thus, during fluid flow, turbulence is created, which tends to increase corrosion attack.
Attempts have been made to improve the quality of joints created by the mating of matched mitered parts. For example, U.S. Pat. No. 3,873,391 shows, in FIGS. 3 and 4, a method of attachment wherein predesigned complementary metered edges are fitted together and secured by means of cement, and thereafter a liquid mixture coating of elastomer resins are applied to the external surfaces of the preformed pipe subsections adjacent to the joint. Thus, the strength of the joint is enhanced. However, in such methods, the corrosion resistance of the joint is not improved.
In larger vessels, typically larger than eighteen inches (18") diameter, it is feasible to put a resin rich interior overlay on the joint created by the method described above. Although such an overlay is a considerable improvement over the method used on smaller diameter pipes, the overlay protrudes from the finished surface of the vessel or pipe part. The overlay therefore creates turbulence in fluid flow and exacerbates corrosion and erosion at the joint. Although the service life of such an "inside overlay" joint is greatly improved over a joint without the inside overlay, the result is still a joint which is quite susceptible to corrosion attack when compared to a smooth molded surface in the vessel or piping system.
In the case of nozzle attachment to pipe or vessels having a diameter of approximately eighteen inches (18") or smaller, it is usually not possible to include a corrosion barrier overlay on the inside of such joint. Thus, the corrosion barrier is interrupted at the joint. Further, corrosion and erosion of joint materials exposes the reinforcement fibers, thus, the entire joint may fail.
Changes in fluid flow direction at nozzle attachment points result in a variety of shear, tensile, and torsion forces due to the hydraulic and fluid dynamic action on the joint. Where RTP parts are joined, one failure mode is for the bond joining the overlay to the primary laminate to fail, resulting in the overlay becoming separated from the primary parts. This phenomenon is known as peeling. Another failure mode is for corrosion and erosion of an initial resin rich layer to expose the reinforcing fibers, allowing further corrosive and erosive attack and thus weakening the joint.
In other known methods for manufacture of FRP vessels, pipe headers, and pipe parts, "stub molds" are mounted directly on the primary mandrel used for producing the main pipe part or vessel. A rounded fillet is created at the juncture of the mandrel and the stub mold by using wax. Resin impregnated fiber is laid down over both the stub mold and the mandrel, and the result is a smooth, rounded corner, with a continuous resinous matrix laminate. Although this results in an equally corrosion resistant and servicable joint as the present invention, it has several disadvantages. The stub mold method results in increased costs, primarily increased utilization time for tooling, and lower productivity of shop labor.
Methods similar to the "stub mold" method have been used for the manufacture of fiber reinforced plastic pipe parts, such as pipe tees. Descriptions may be found in the U.S. Pat. No. 4,601,496, or in earlier variations as shown in U.S. Pat. No. 4,106,797 and U.S. Pat. No. 3,765,979. Fundamental to each of these methods is the simultaneous use of a mandrel on each axis; i.e., a mandrel for the main part and another mandrel for the branch part. A solid one-piece, two-axis, expendable mandrel is disclosed in U.S. Pat. No. 4,601,496, in FIG. 1.